Electrostatic discharge protection device

ABSTRACT

An electrically conductive network which, at one end, has a low-resistance connection with a user&#39;s hand and, at the other end, has a high-resistance connection with ground. This protects electrical equipment such as a telephone keyset from electrostatic discharge and also protects the user from unpleasantly high discharge currents.

BACKGROUND OF THE INVENTION

The invention relates to protective devices, and more particularlyrelates to protective devices which protect electrical equipment andthose who use it from electrostatic discharge. In its most inmmediatesense, the invention relates to an electrostatic discharge protectiondevice for use in telephone keysets.

Electrostatic discharge is often encountered with, e.g., telephonekeysets and other electronic devices that are used in the home or in theoffice. Where a user accumulates an electrostatic charge by walking oncarpeting or by sitting on upholstered furniture, the charge may bedissipated through, e.g., a telephone keyset when the user picks up thereceiver. Because relatively high electrostatic voltages can be builtup, electrostatic discharge can be painful because the discharge currentcan be relatively high. Furthermore, such high currents can generatestrong electromagnetic fields; these can induce voltages in sensitiveelectronic equipment and damage it.

Originally, attempts to reduce the current produced during electrostaticdischarge were directed to increasing the dielectric strength of thehousing of, e.g. telephone keysets. Such results can be obtained by,e.g. using intermediate layers of insulation and thereby increasing thedistance between the sensitive electronic circuitry and the user's hand.However, such attempts have proved unsatisfactory because highelectrostatic voltages require entirely new housing structures.

It would be advantageous to provide a device which would protect both apiece of electronic equipment and the user of such equipment fromelectrostatic discharge.

SUMMARY OF THE INVENTION

In accordance with the invention, the equipment to be protected isprovided with an electrically conductive network which is so locatedthat the user will make electrical contact with it when he grasps theapparatus. The network includes a low resistance in series with a highresistance. The high resistance is grounded, and the user makeselectrical contact with the low resistance.

Because the electrostatic discharge takes place through the highresistance, the discharge current is limited to a relatively low valueand this makes the discharge unnoticeable to the user. Because thedischarge current is low, the resulting electromagnetic field isrelatively weak, which considerably reduces the likelihood of damagecaused by induced voltages. Thus, both the user and the equipment areprotected.

Advantageously, the conductive network can be formed by a layer placedon the inside of the equipment housing and forming a connection withthose parts which a user grasps to use the equipment. Thus, for example,in the case of a telephone keyset, the electrically conductive materialcan be applied on one of the two housing parts of the handset in such away that it extends to the encircling boundary between the parts andthus makes an electrical connection with the operator. This layer canalternatively be formed by a metal foil or an electrically conductivesynthetic foil.

The high resistance can be formed by a discrete structural element or bya resistive paste applied to a carrier. The resistive paste can, forexample, be applied on a carrier foil by a screen printing processpermitting cost-effective mass production. This carrier can be providedwith recesses with contact devices, which can be mounted on thestructural elements to be protected. Using the carrier foil, the bondingdevices can be fashioned as lobes projecting into the recess, connectedwith each other and with the high resistance, which are installed in thetelephone handset, for example at the metal housing of the speaker.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary and non-limiting preferred embodiments of the invention areshown in the drawings, in which:

FIG. 1 shows in partial section a side view of a handset incorporating apreferred embodiment of the invention; and

FIG. 2 shows a top view of a preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the present embodiment, protection for a telephone handset and forthe person operating it are illustrated, but the invention is notlimited to this application.

A handset 2 is formed from two housing half-shells 1 and 4. Thehalf-shell 1 has a conductive paste applied to all or a part of itsinner surface. This paste can, for example, be applied using an adhesivespray process. In this process, the conductive paste is so applied thatit spreads to the circumferential boundary line 14 which separates thehalf-shells 1 and 4, so that through the paste an electrical connectionwith the operator (not shown) may be established. The conductive paste 3also coats the ports 5 in the half-shell 1, which ports 5 are located inthe speaker region so that sound can be transmitted to the user's ear.This conductive paste which has a relatively low resistance can bereplaced, for example, by an appropriate metal foil or conductivesynthetic foil, the foil in the last mentioned case serving a decorativefunction at the boundary line 14.

Within the handset 2 an additional resistive foil 6 is fastened in thatregion of the half-shell 1 where the conductive paste 3 is applied. Theresistive foil 6, which can, for example, be produced by screenprinting, has a circular cut-out 9 corresponding approximately to theouter diameter of the metal housing 7 of the speaker 8. Into the centerof this cut-out 9 point several lobes 10 which are printed withconductive paste and which are connected with each other in the shape ofa ring. These lobes 10 are connected over a zig-zag high resistanceresistor 11. Resistor 11 consists of resistive paste the resistor havingan ungrounded end at terminal 12 and a grounded end at lobes 10 which isgrounded through an electrical path (not shown). Bonding of theresistive foil 6 with the speaker 8 takes place by attaching the foil 6with the area having the cut-out 9, whereby the lobes 10 are placed onand electrically connected to the metal housing 7. The electricalconnection of the resistive foil 6 with the conductive paste 3 withinthe housing part 1 of the handset 2 takes place through the terminal 12and screw 13. Discharge of an electrostatic charge then takes placethrough a high-resistance path 6 through metal housing 7 to ground.

Those skilled in the art will understand that changes can be made in thepreferred embodiments here described, and that these embodiments can beused for other purposes. Such changes and uses are within the scope ofthe invention, which is limited only by the claims which follow.

What is claimed is:
 1. A device for use with electronic apparatus todissipate electrostatic charges which have accumulated on users of thedevice, comprising:means for establishing a low resistance connectionwith the user; and a high resistance means having a grounded end and anungrounded end, the ungrounded end being electrically connected to saidestablishing means.
 2. The device of claim 1, wherein said establishingmeans comprises an electrically conductive layer which is located insidethe apparatus and which is electrically connected to electricalcomponents therein.
 3. The device of claim 2, wherein the layercomprises an electrically conductive paste.
 4. The device of claim 2,wherein the layer comprises metal foil.
 5. The device of claim 2,wherein the layer comprises electrically conductive synthetic foil. 6.The device of claim 1, wherein the high resistance means is a discretecomponent.
 7. The device of claim 1, wherein the high resistance meanscomprises a carrier to which a resistive paste is applied.
 8. The deviceof claim 7, wherein the carrier has an opening shaped to receive aconductive electrical component of the apparatus to be protected, andwherein the carrier has regions which serve as terminals makingelectrical contact with said conductive electrical component.
 9. Thedevice of claim 1, wherein the apparatus comprises a handset and saidestablishing means comprises an electrically conductive material whichis so located as to make electrical contact with a user who grasps thehandset.
 10. The device of claim 9, wherein the handset has a speakerand ports transmitting sound from the speaker and wherein theelectrically conductive material coats said ports.